Aqueous dispersions of vinyl ester polymers

ABSTRACT

AQUEOUS DISPERSIONS OF VINYL ESTER POLYMERS HAVING A VISCOSITY INDEX OF NOT GREATER THAN ABOUT 1.8, AND A PROCESS FOR MAKING THEM, ARE PROVIDED. FILMS CAST FROM THESE DISPERSIONS POSSES GOOD WATER RESISTANCE AND STABILITY. THE DISPERSIONS CONTIN AS A PROTECTIVE COLLOID PARTIALLY HYDROLYZED POLYVINYL ALCOHOL, DERIVED FROM POLYVINYL ACETATE, HAVING AN AVERAGE VINYL ACETATE CONTENT OF 5 TO 7 MOLE PERCENT AND CONSISTING ESSENTIALLY OF PPOLYVINYL ALCOHOL MOLECULES ESSENTIALLY NONE OF WHICH CONTAINS MORE THAN ABOUT 16 MOLE PERCENT VINYL ACETATE. THE POLYVINYL ALCOHOL MAY CONSIST OF A BLEND OF TWO OR MORE POLYVINYL ALCOHOLS EACH OF WHICH HAS AN EVERAGE POLYVINYL ACETATE CONTENT DIFFERENT FROM THE AVERAGE OF THE BLEND; HOWEVER, THE AVERAGE POLYVINYL ACETATE CONTENT OF ANY POLYVINYL ALCOHOL BLENDED SHOULD NOT EXCEED ABOUT 16 MOLE PERCENT.

United States Patent Office 3,827,996 Patented Aug. 6, 1974 U.S. Cl.260-29.6 WB 6 Claims ABSTRACT OF THE DISCLOSURE Aqueous dispersions ofvinyl ester polymers having a viscosity index of not greater than about1.8, and a process for making them, are provided. Films cast from thesedispersions possess good water resistance and stability. The dispersionscontain as a protective colloid partially hydrolyzed polyvinyl alcohol,derived from polyvinyl acetate, having an average vinyl acetate contentof to 7 mole percent and consisting essentially of polyvinyl alcoholmolecules essentially none of which contains more than about 16 molepercent vinyl acetate. The polyvinyl alcohol may consist of a blend oftwo or more polyvinyl alcohols each of which has an average polyvinylacetate content different from the average of the blend; however, theaverage polyvinyl acetate content of any polyvinyl alcohol blendedshould not exceed about 16 mole percent.

CROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of my copending application Ser. No. 850,649, filedAug. 15, 1969, now abandoned.

BACKGROUND OF INVENTION Aqueous dispersions containing vinyl esterpolymers, e.g., vinyl acetate polymers or copolymers, are well known andhave found particular utility in adhesive applications. For such uses,it has been customary to polymerize or copolymerize vinyl acetate in anaqueous medium containing polymerization initiators and water-solubleprotective colloids. Many of the problems associated with thepreparation of useful dispersions have been discussed in a variety ofpatents, among which the following are included: U.S. Pat. 2,227,163issued to Werner Starck and Heinrich Freudenberger on Dec. 31, 1940,U.S. Pat. 2,398,- 344 issued to Heny M. Collins and Mogens Kiar on Apr.16, 1946, US. Pat. 2,388,600 issued to H. M. Collins on Nov. 6, 1945,U.S. Pat. 2,892,802 issued to Eugene P. Budewitz on June 30, 1959, U.S.Pat. 3,094,500 issued to Stedman C. Herman on June 18, 1963 and CanadianPat. 812,285. As is evident from reading these patents, a major problemin providing a suitable dispersion based on a vinyl acetate polymer hascentered on using the appropriate protective colloid. While, as thesepatents illustrate, water-soluble partially or completely hydrolyzedpolyvinyl acetates have solved many of the problems associated withfinding an appropriate protective colloid, there has not yet beenprovided a completely satisfactory vinyl acetate polymer dispersion foradhesive applications.

The most important properties which an adhesive, based on a vinyl esterpolymer dispersion, should have are stability, both during and afterpreparation, water resistance of the adhesive after it has beendeposited from the dispersion, and a satisfactory viscosity index. Thefirst of these properties, i.e., stability, is by and large possessed bythe customary vinyl ester polymer dispersions. Moreover, waterresistance is generally achieved by including post-reactive ingredientsin the dispersions. However, a dispersion with a satisfactory viscosityindex, while being stable and giving water-resistantfilms has beenlacking.

The viscosity index is the ratio of the Brookfield viscosity of thedispersion measured at 6 rpm. to the Brookfield viscosity of thedispersion measured at 60 r.p.m. An ideal viscosity index is 1. Such anindex evidences the fact that the viscosity of the dispersion isindependent of shear rate. For practical purposes, the viscosity indexis important since, during application of the adhesive dispersion bycustomary application machines, the dispersion is sheared at varyingrates. If dispersion yiscosity is independent of shear rate, a uniformapplied adhesive thickness can be achieved without the necessity forelab orate controls. 1

SUMMARY OF THE INVENTION According to the present invention, there isprovided a process for preparing an aqueous dispersion of vinyl esterpolymer possessing the properties of stability, water resistance, evenin the absence of a post-reactive ingredient, and a viscosity index ofnot greater than about 1.8. The aqueous dispersion consists essentiallyof dispersed vinyl ester polymer and a stabilizing amount of polyvinylalcohol having an average residual vinyl acetate content of 5 to 7 molepercent. Polyvinyl alcohol having this vinyl acetate content may be usedor polyvinyl alcohols having average vinyl acetate contents of up toabout 16 mole percent may be blended to produce an average of 5 to 7mole percent.

DESCRIPTION OF PREFERRED EMBODIMENTS As used herein, the term consistingessentially of means that the named ingredients are essential; however,other ingredients which do not prevent the advantages of the inventionfrom being realized can also be included. Thus, while the vinyl esterpolymer and the polyvinyl alcohol are recited as being essentialingredients, other ingredients commonly found in vinyl ester polymerdispersions can also be included in the presently claimed dispersions.Among others, such ingredients include initiators and activators notconsumed in the polymerization process, buffers, preservatives, andfillers. Also, the present dispersions can contain water-solublesurfactants. However, with the exception of small amounts of nonionicsurfactants as discussed hereafter, the presence of surfactants is notordinarily desirable, especially where a high degree of water resistanceis required.

Methods of preparing vinyl ester polymer dispersions are well known andany of the customary techniques can be used; however, a preferred methodis disclosed in U.S. Pat. 3,534,009 to Aleksander Beresniewicz andWilliam H. Todd, and assigned to the assignee of the presentapplication. The term vinyl ester polymer dispersions is intended toinclude aqueous polymer dispersions wherein the polymer contains atleast about 60 weight percent of polymerized vinyl ester. While vinylacetate is the preferred vinyl ester, other vinyl esters such as vinylformate, vinyl propionate, vinyl butyrate as Well as higher vinyl esterssuch as vinyl stearate and vinyl palmitate are also useful. The vinylester polymer can be a homopolyiner of a particular vinyl ester or itcan be the product ofthe copolymerization of a vinyl ester with anothervinyl'ester or with one or more additional ethylenicallyunsaturatedcopolymerizable monomers. Among others, useful additionalcopolymerizable monomers include acrylates such as methyl acrylate,ethyl acrylate, Z-ethylhexyl acrylate, methacrylates such as methylmethacrylate, ethyl methacrylate, 2-ethylhexyl methacrylate, carboxylicacids such as acrylic acid and methacrylic acid, other vinyl compoundssuch as vinyl chloride and vinylidene chloride, and olefins such asethylene and the butenes.

Of the above copolymerizable monomers, ethylenehas been found to beparticularly suitable. In combination With vinyl acetate, ethylene isordinarily copolymerized in an amount of about to 40 percent by weight,and preferably to weight percent. A particularly useful method forpreparing vinyl acetate/ethylene copolymer dispersions involves anaqueous dispersion technique wherein the polymerization is accomplishedunder an ethylene pressure of about 100 to 1000 p.s.i.g. and at atemperature of about 40 to 80 C. Particularly useful catalyst systemsfor this polymerization include persulfates or peroxides used with orwithout reducing agents such as bisulfites or formaldehyde sulfoxylates.

In order to prepare the present dispersions, vinyl ester polymerizationis carried out in an aqueous medium containing'as a protective colloidpartially hydrolyzed polyvinyl alcohol, derived from polyvinyl acetate,which has an average residual vinyl acetate content of 5 to 7 molepercent. Dispersions wherein the polyvinyl alcohol contains largeraverage amounts of residual vinyl acetate have poor viscosity indexesand yield films with poor water resistance. The use of polyvinylalcohols with an average residual acetate content of less than 3 molepercent resuit in dispersions with deficient stability. The polyvinylalcohol can be present as a blend of two different polyvinyl alcohols,i.e., a partially hydrolyzed polyvinyl alcohol having a vinyl acetatecontent of from about 12 to 16 mole percent and an essentiallycompletely hydrolyzed polyvinyl alcohol having a vinyl acetate contentof from about 0.5 to 1 mole percent.

The polyvinyl alcohol content of the dispersion will generally rangefrom about 2 to 10 percent by weight, based on the weight of the vinylester polymer; however, best results are obtained when the polyvinylalcohol concentration is about 3 to 6 percent by weight.

When two different grades of polyvinyl alcohol are used as a blend, theratio of these polyvinyl alcohols in the blend is determined by thedegree of hydrolysis and the molecular weight of each grade. It isgenerally advantageous to produce dispersions in the viscosity range of1000 to 3000 centipoise, as determined by a Brookfield viscometer of 60r.p.m. The molecular weights of the polyvinyl alcohols are so chosen asto give, under given polymerization conditions, the desired emulsionviscosity. When blending the polyvinyl alcohols of two different degreesof hydrolysis, the following considerations are important:

l. The average degree of hydrolysis of the blended polyvinyl alcohol issuch that about 5 to 7 mole percent of vinyl acetate is unhydrolyzed;

2. The blend should not have a substantial amount of polyvinyl alcoholcontaining more than about 16 mole percent of unhydrolyzed vinylacetate.

Dispersions of the present invention have the advan tages of goodstability, high solids content and also the advantage that films castfrom the dispersion possess good water resistance even without the useof post-additives. Solids content will range from 40 to 60 percent byweight and will usually be in the range of 55 to 60 percent. Ofparticular advantage is the fact that very good viscosity index valuescan be obtained at a high solids level, as seen in Example 4, Table I.

In the examples which follow, stability of the dispersion is judgedvisually by dispersion appearance, i.e., on a gritty or smooth textureand whether separate layers are visible after 6 months storage. Waterresistance is determined by the Weyerhaeuser soaking under stress test.In this test, wet strength kratt paper is used to make l-inch overlapjoints. The emulsion is drawn down with a No. 20 RDS wire-wound rod onone sheet of paper, and another sheet is overlaid. The assembly isair-dried at ambient temperature for 24 hours. Then l-inch wide stripsare cut and immersed in water, at ambient temperature, with a 350-gramweight attached in a shear test. This gives a shearing force of 350grams per square inch of the lap joint. The test is rated as fail ifdelamination occurs in less than 24 hours.

The invention can be further understood by the f0llowing examples inwhich parts and percentages are by weight unless otherwise indicated.

EXAMPLE 1 The following ingredients were charged into a stirred 5-gallonpressure vessel:

189 grams of an 88% hydrolyzed, low molecular weight polyvinyl alcohol,

111 grams of a 99.5% hydrolyzed, medium molecular weight polyvinylalcohol,

both dissolved in 3300 grams of distilled water,

235 grams of a 6.1% solution of zinc formaldehyde sulf oxylate in water3860 grams of vinyl acetate.

After closing the vessel and purging it with nitrogen, the contents wereheated to 60 C. and the ethylene pressure was brought to 540 p.s.i. Thepolymerization was initiated by adding 60 cc. of a 0.6% H 0 solution inwater.

When the polymerization started, as indicated by observance of anexotherm, the separate continuous feeds were added to the vessel.

Feed 1 4080 grams of vinyl acetate, added over a period of minutes.

Feed 2 28 grams of 88% hydrolyzed, low molecular weight, polyvinylalcohol 111 grams of 99.5% hydrolyzed, medium molecular weight polyvinylalcohol 37 grams of Triton X100, (a nonionic surfactant, sold by Rohm &Haas Co.)

All dissolved in 2520 grams of distilled water.

This Feed 2 was added over a period of minutes. Ethylene pressure wasmaintained at 600 p.s.i., and, by addition of initiator solution, thepolymerization temperature was maintained at 60 C. The reaction exothermdied down after about 4 hours, at which time the ethylene pressure wasreduced to atmospheric. At this point, the residual monomer content wasabout 3%. Thereafter, 100 grams of the above-identified initiatorsolution were added to the emulsion. After maintaining the emulsion forabout 30 minutes at 40 C., the residual vinyl acetate monomer contentwas less than 1.0%, based on the weight of the emulsion.

The emulsion had a solids content of 56.8% with the copolymer (VAc/E)comprising 85.1% vinyl acetate and 14.9% ethylene, a Brookfieldviscosity at 6 r.p.m. of 3400 cps., a Broolcfield viscosity at 60 r.p.m.of 2140 and a viscosity index of 1.6. The mixture of polyvinyl alcohols(PVA) used had an average mole percent of vinyl acetate of 6.2%.Stability of the emulsion was rated good and'the water resistance of afilm cast from the emulsion was rated very good.

EXAMPLES 2 TO 5 Following the procedure of Example 1, various emulsionswere prepared containing a copolymer of 86% vinyl acetate and 14%ethylene, and also varying grades and amounts of polyvinyl alcohol. Thecopolymers of Examples 2, 3 and C contained 0.07, 0.05 and 0.06%,respectively, of sodium vinyl sulfonate interpolymerized therein. InExample 6 the general procedure of Example 1 was repeated except that asingle polyvinyl alcohol having an average residual vinyl acetatecontent of about 6.1 mole percent was used.

The emulsions and their properties are shown in Table I which follows:

2. The process of claim 1 wherein the vinyl ester is vinyl acetate.

TABLE I.BLENDS OF POLYVINYL ALCOHOLS OF VARYING RESIDUAL VINY L ACETATECONTENT IN VINYL ACETATE] EIHY LENE COPOLYMER EMULSIONS AND THE EFFECTON STABILITY AND CAST FILM WATER RESISTANCE Wt. percent based on theVAc/E copolyrner Average Emulsion Properties Polyvinyl alcohol with thefollowing mole Sodium mole Water percent of residual VAc vinyl percentWt. Brookfleld viscosity atresist sulfo- Snr- VAc in percent ance of Ex17 15 12 0. 1 5. 5 3. 8 2. 1. 0 0. nate fnctant the PVA solids 6 r.p.m.60 r.p.m. B=n6ln60 Stability film 12. 0 55. 0 3, 650 1, 400 2. 4 0. 076. 8 54. 9 2, 750 1, 550 1. 8 0.05 5. 9 55. 8 1, 400 980 l 4 *0. 2 4. 656. 5 706 700 1. 0 3. 7 59. 5 1, 300 1, 200 l. 1 C4 '1. 4 3. 6 57. 2 1,360 1, 300 1. 0 C5 1.5 5.8 53.1 520 400 1.3 4 1.3 5.9 59.8 1,600 1,0601.5 4. 7 56. 5 3, 200 2, 500 1. 3

5 2. 0 3. 0 5.8 56.8 6, 600 3, 720 1. 8 G1 1.5 1 5 1.0 0.5 4.0 58.0 oPoor 6 5.0 6.1 56.5 6,600 3,720 1.8

"Tritou" X-IOO-a nonionic surfactant, octyl phenoxy polyethoxy ethanol,manufactured by Rohm and Haas Co. "Aerosol MA-80-an anionic surfactant,sodium dlhexylsuliosuccinate, manufactured by American Cyanamid.'"Poor-delamlnated in a few hours. Good-passed the required 24 hours; V.goodpassed over 65 hours.

Control C shows the results obtained with a typical vinyl acetate basedemulsion containing an 88% hydrolyzed polyvinyl alcohol as a protectivecolloid. Examples 2 and 3 show that stability falls off when too muchfully hydrolyzed (0.5 mole percent vinyl acetate) polyvinyl alcohol isused; however, in this latter case, a nonionic surfactant helpsstability as shown in Example 1. In Control 0,, results are good but toomuch undesirable surfactant is needed. Control C shows that a readilywatersoluble anionic surfactant helps stability, but adversely affectswater resistance. Examples 4 and 5 and Controls 0,, and C show that theaverage mole percent of vinyl acetate in the polyvinyl alcohol must becarefully chosen, e.g., in C one of the polyvinyl alcohols used had atoo low degree of hydrolysis, and this affects both stability and waterresistance. Example 5 shows that it is not necessary to mix two gradesof polyvinyl alcohol with extremes in unhydrolyzed vinyl acetate to getgood stability and water resistance. Example 6 shows that good resultsare obtained by utilizing a single polyvinyl alcohol rather than ablend.

I claim:

1. A process for preparing a stabilized aqueous dispersion of a vinylester polymer, said dispersion having a viscosity index of not greaterthan about 1.8, comprising copolymerizing, in an aqueous medium, vinylester and ethylene in the presence of partially hydrolyzed polyvinylalcohol derived from polyvinyl acetate, said polyvinyl alcohol having anaverage vinyl acetate content of about 5 to 7 mole percent andconsisting essentially of polyvinyl alcohol molecules containing no lessthan about 0.5 mole percent nor more than about 16 mole percent vinylacetate, said polyvinyl alcohol being present in the amount of about 2to weight percent based on the weight of the vinyl ester.

3. The process of claim 1 wherein the polyvinyl alcohol is a blend oftwo or more polyvinyl alcohols each of which 'has an average vinylacetate outside the range of 5 to 7 mole percent but within the range of0.5 to 16 mole percent.

4. The process of claim 3 wherein the polyvinyl alcohol is comprised ofa blend of polyvinyl alcohol having a vinyl acetate content of about 0.5to 1 mole percent and polyvinyl'alcohol having a vinyl acetate contentof about 12 to 16 mole percent.

5. The dispersion produced by the process of claim 1.

6. The dispersion produced by the process of claim 3.

References Cited UNITED STATES PATENTS 3,532,658 10/1970 Gintz 260-296 R3,010,929 11/1961 Jones 260-29.6 3,094,500 6/ 1963 Merman 260-2963,197,429 7/ 1965 Baatz 260-296 3,213,051 10/ 1965 Pierce 260-2963,634,349 1/ 1972 Goi et al. 260-17 3,644,257 2/ 1972 Nickerson et al.260-29.6 WB 3,661,696 5/1972 Knutson 161204 OTHER REFERENCES Skeist,Handbook of Adhesives, pp. 371-2 Van Nostrand-Reinhold 1962.

WILLIAM SHORT, Primary Examiner E. A. NIELSEN, Assistant Examiner US.Cl. X.R.

